In place or in addition to the conventional storage apparatus using a hard disk as a memory medium, a storage apparatus called SSD (solid state drive), in which a semiconductor memory is used as a memory medium, is currently in wide use. As the SSD semiconductor memory, a flash memory using a cell transistor having a floating gate is generally used today.
The flash memory has a program operation in which a pulse is applied to the cell transistor to inject or charge electrons into the floating gate, and an erase operation in which a high voltage is applied to a substrate to extract or discharge electrons in the floating gate. This produces the problem of a deterioration in the flash memory by the passage of electrons due to a tunnel effect through the insulating film between the semiconductor substrate and the floating gate of the cell transistor. This deteriorated insulating film brings about an increased pulse count needed for the program operation.
As a result, it is needed to periodically diagnose the SSD storage apparatus mounted in a computer to check whether or not the deterioration occurs. On detection of deterioration, a continued use of such an SSD storage apparatus may cause a computer malfunction.
SSD storage apparatus is described in International Patent Application WO 2006/025083, the official gazette of the Japanese Laid-open Patent Publication No. 2004-054966 and No. 2008-176826.